An optical fiber is formed by drawing an optical preform on a drawing tower. An optical preform generally includes a primary preform, which itself includes a silica tube of pure or doped silica in which doped and/or pure silica layers have been successively deposited. (These layers become the optical fiber's inner cladding and a central optical core.) The primary preform is typically overcladded or sleeved to increase its diameter, thereby forming a final preform that, on a drawing tower, can be drawn into optical fiber. In this context, the term “inner cladding” refers to the optical cladding formed inside the silica tube, and the term “outer cladding” refers to the optical cladding formed on the outside of the silica tube.
The homothetic optical fiber drawing operation includes placing the optical preform vertically in a tower and drawing an optical fiber strand from one end of the optical preform. For this purpose, high heat is applied to an end of the preform until the silica softens. To achieve an optical fiber of a particular diameter, the drawing speed and temperature are continuously controlled during the drawing operation.
An optical fiber conventionally includes an optical core, which functions to transmit and, optionally, to amplify an optical signal, and an optical cladding, which functions to confine the optical signal within the optical core. For this purpose, the refractive index of the optical core (nc) is greater than the refractive index of the optical cladding (ng) (i.e., nc>ng). As is well known, the propagation of an optical signal in a single mode optical fiber decomposes into a fundamental mode guided in the optical core and into secondary modes (i.e., cladding modes) guided over a certain distance in the optical core-optical cladding assembly.
A silica tube may be made according to chemical vapor deposition (CVD), such as described in U.S. Pat. No. 3,907,536, which is hereby incorporated by reference in its entirety. This kind of deposition is performed by injecting gas mixtures in a glass substrate tube and then ionizing the gas mixtures. As will be known by those having ordinary skill in the art, chemical vapor deposition encompasses modified chemical vapor depositions (MCVD), furnace chemical vapor depositions (FCVD), and plasma-enhanced chemical vapor depositions (PCVD). The PCVD method is described, for example, in U.S. Pat. No. 4,145,456, which is hereby incorporated by reference in its entirety.
After depositing the glass layers corresponding to the optical core and the inner cladding, the silica tube is closed upon itself in a so-called collapsing operation to obtain a primary preform (i.e., a silica rod). This primary preform is then overcladded to increase its diameter, typically using relatively inexpensive natural silica grain.
U.S. Patent Application Publication No. 2003/0019245, which is hereby incorporated by reference in its entirety, discloses a method of purifying silica and depositing the purified silica onto an optical fiber preform. The overcladding may be carried out by plasma deposition in which the natural silica grain is projected on the primary preform and, via a plasma torch, are fused at a temperature of about 2,300° C. The natural silica grain is vitrified on the periphery of the primary preform to form an outer optical cladding. During the overcladding process, the primary preform is caused to rotate around its longitudinal axis and the plasma torch and/or the primary preform move longitudinally with respect to each other. Such rotational and translational movement facilitates uniform silica deposition over the periphery of the primary preform.
To effect preform overcladding, the primary preform must be connected to an overcladding device, such as a glass-working lathe, and rotated around its longitudinal axis. Typically, this connection is carried out by welding the ends of the primary preform to silica bars that are positioned in the overcladding device. The overcladding operation is generally carried out in a closed cabin with a controlled atmosphere so as to provide protection against electromagnetic perturbations and the evolvement of ozone emitted by the plasma torch.
For example, Japanese Patent Application No. JP 57111255, which is hereby incorporated by reference in its entirety, discloses welding a colored quartz disc coaxially between two quartz preform rods to form a discriminative layer (or indicator). During subsequent drawing of optical fibers, this colored indicator facilitates the detection of the transition between the respective preform rods.
U.S. Pat. No. 6,178,779, which is hereby incorporated by reference in its entirety, discloses a method of assembling two optical fiber preforms end-to-end by heating the ends of the preforms and pressing the respective heated ends into contact.
German Patent Application No. DE 912622, which is hereby incorporated by reference in its entirety, discloses a method of assembling two glass rods having different melting points by using intermediate pieces of glass.
U.S. Pat. No. 4,407,667, which is hereby incorporated by reference in its entirety, discloses a continuous process of manufacturing optical fibers by inserting preforms one after the other in a drawing tower and welding them end-to-end in a welding station.
U.S. Pat. No. 6,305,195, which is hereby incorporated by reference in its entirety, discloses isothermal-plasma-torch techniques for welding preforms end-to-end.
European Patent Application No. EP 1,690,836 and its counterpart U.S. Patent Application Publication No. 2007/0147748, which is hereby incorporated by reference in its entirety, disclose a method for connecting optical fiber preforms by bringing the respective preform ends close together, then heating and connecting the respective preform ends.
FIG. 1 illustrates a typical installation for overcladding an optical fiber preform. A primary preform 100 is provided with a first end welded to a first silica bar 215, which is attached to a first glass-working lathe support 205 by chucks 225.
In this regard, the first end of the primary preform 100 is welded by hand to the first silica bar 215 using, for example, a H2/O2 burner (e.g., a blow torch). Such a blow-torch welding operation is time consuming and costly because it is performed by hand (i.e., by an operator). This hand welding should be performed under an exhaust hood for health and safety reasons.
Thereafter, the primary preform 100 is placed on a glass-working lathe, which includes a second silica bar 210 that is affixed by chucks 220 to a second glass-working lathe support 200. The glass-working lathe drives into rotation both (i) the first silica bar 215 and the affixed primary preform 100 and (ii) the second silica bar 210. A mandrel (not shown) facilitates the rotation of the second silica bar 210. As the primary preform 100 rotates around its longitudinal axis, it is welded by hand to the second silica bar 210 using, for example, the aforementioned H2/O2 burner.
The silica used for the silica bars 210, 215 is typically low-cost, undoped silica that contains impurities (e.g., dust particles or water). On the other hand, the silica of the primary preform 100 may be doped—perhaps highly doped if the preform is intended to produce single-mode optical fibers or chromatic dispersion compensating optical fibers. The difference in doping between the silica of the primary preform 100 and the silica of the silica bars 210, 215 causes notable performance differences between both materials, notably with respect to viscosity and thermal expansion.
Consequently, the welding of the primary preform 100 directly to the supporting silica bars 210, 215 of the glass-working lathe supports 200, 205 is delicate, requiring care and precision to form a sufficiently strong and accurate weld. For instance, the first end of the primary preform 100 must be carefully welded to the first silica bar 215 with a burner (e.g., a blow torch). With slow and gradual heating, it is possible to produce a solid weld. This operation, however, is costly in both time and productivity.
Moreover, hand welding the primary preform 100 to the first silica bar 215 (or to the second silica bar 210) is subject to human error. The use of a hand-held torch can cause sudden heating or cooling of the silica when the burner (e.g., a blow torch) is directed at or removed from the welding area. This sudden heating or cooling, typically the result of poor welding technique or other human error, exacerbates the differences in thermal behavior, (i.e., difference in expansion or shrinkage between the two materials that are welded). This can yield a low-quality weld between, for example, the first end of the primary preform 100 and the first end of the silica bar 215 (i.e., a brittle weld).
In addition, during the welding process both ends of the primary preform 100 and the ends of the respective silica bars 210, 215 are heated and softened. As noted, contaminants that are present in the silica bars 210, 215 may diffuse into the ends of the primary preform 100. This diffusion of contaminants changes the composition of the primary preform 100 (i.e., after welding, the end of the primary preform 100 is no longer of the same composition as before welding). Consequently, a primary-preform outer end that is welded can no longer be used for drawing optical fiber. In other words, using the foregoing process, a part of the primary preform 100 is unavailable for fiber drawing.
Likewise, it is known to weld together optical fiber preforms in which both of the optical preforms to be welded end-to-end are generally of the same nature, (i.e., intended for drawing the same kind of optical fiber). In such methods, each optical preform consists of silica containing substantially the same dopant concentrations.
The aforementioned U.S. Pat. No. 6,178,779, for example, describes a method for end-to-end assembling two optical fiber preforms. Both optical preforms are mounted on a glass-working lathe in a longitudinal alignment and rotated. Then, a burner uniformly heats each preform end in an axial reciprocal movement. The heated preform ends are then brought together and welded.
U.S. Pat. No. 6,098,429, which is hereby incorporated by reference in its entirety, describes an optical fiber-drawing method in which the optical fiber preforms are drawn continuously. Preforms are welded end-to-end in the drawing tower by a servo-controlled laser in a way that reduces the area affected by the welding.
Such welds of optical fiber preforms are carried out on silica bars having substantially the same properties in terms of viscosity and thermal expansion. The optical preforms are intended for drawing the same kind of optical fiber and so have the same dopant concentrations. The weld between adjacent preforms is therefore generally clean and solid.
It is often necessary, however, to weld silica bars having very different properties (e.g., chemical compositions). For example, a primary preform, often containing dopants in various concentrations, may have to be secured to an undoped silica bar of a glass-working lathe support. Alternatively, two preforms with different dopant concentrations may have to be welded together (e.g., welding a highly doped preform with a slightly doped preform).
Welding silica bars of different compositions is rife with complications. The property differences of the materials to be welded can cause weld brittleness. Consequently, weld failure and breakage is unacceptably common. During overcladding operations, for example, weld failure can lead to the destruction of an expensive primary preform. Therefore, there is a need for an efficient method for welding a primary preform to a silica bar that has different properties in a way that reduces the risk of preform breakage during overcladding operations.